It is known practice to dye keratin fibers, for example, human keratin fibers such as the hair, with dyeing compositions comprising direct dyes. These compounds may be colored, and coloring, molecules having an affinity for the fibers. It is known practice, for example, to use direct dyes of nitrobenzene type, anthraquinone dyes, nitropyridines, and dyes of azo, xanthene, acridine, azine, and triarylmethane type.
Commonly these dyes are applied to the fibers, optionally in the presence of an oxidizing agent if a simultaneous fiber lightening effect is desired. When the leave-in time has elapsed, the fibers may be rinsed, optionally washed, and dried.
The colorations which result from the use of direct dyes are temporary or semi-permanent colorations, because the nature of the interactions which bind the direct dyes to the keratin fiber, and their desorption from the surface and/or the core of the fiber, are responsible for their relatively low tinctorial strength and relatively poor wash resistance and/or perspiration resistance.
European Patent Application No. 1 377 263 discusses employing direct cationic diazo dyes comprising two cationic heterocyclic groups. These compounds, although representing an advance in the art, give dyeing results which nevertheless remain capable of improvement.